Recovery of maleic anhydride



Dec. E3,

TAT

RECOVERY OF MALEIC ANHYDRIDEE Elton B. Purmett, Buffalo, N. Y., assignor to National Aniline & Chemical Company, Inc., New York, N. Y., a corporation of New York No Drawing. Application November 30, 1936, Serial No. 113,534

30 Claims.

erally practiced, vapors of an organic compound,

as for example, benzene, naphthalene, anthracene, phenanthrene, acenaphthene, phenol, tar acids, their derivatives, etc, are mixed with air or other oxidizing gas, the mixture is' brought into contact with an omdatlon catalyst under controlled reaction conditions, and the resulting reaction mixture is cooled to condense the partial oxidation product thereby formed. The reacticn mixtures resulting from the catalytic oxidathan generally contain, in addition to the desired products, a number of other substances in various amounts which sometimes condense with the desired products, thereby contaminating them, so that a subsequent purification of the desired products is generally required before they can be employed for the usual purposes.

in the course of the catalytic oxidation, water is formed as one of the by-products of the reaction and it frequently condenses with some of the partial oxidation products, particularly at low temperatures. If, in addition, the desired product is reactive toward water at the temperature of condensation, as for example, maleic anhydride, it has been the practice to absorb the gaseous reaction mixture in water, and then dehydrate the resulting product by a separate process.

An object of the present invention is to provide a method of recovering organic partial oxidation products which are reactive with water, directly from mixtures containing them together with water, while preventing reaction with the water.

A further object of the invention is to provide method of recovering maleic anhydrlde directly in the form of the n'oydride from a mixture containing maleic anhydride and water vapors.

Another object of the invention is to provide a method of recovering maleic anhydride in an efflcient and economical manner directly in the form oi the anhydride from a gaseous mixture contain: ing'the same, more particularly, from a gaseous mixture aiso containing water, and especially tram a gaseous reaction mixture resulting from the catalytic partial oxidation of an organic compound in the vapor phase.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention will be described in connection with the recovery of maleic anhydride from gaseous reaction mixtures resulting from the catalytic partial oxidiation of an organic compound in the vapor phase. The invention is not limited thereto, however, and various changes may be made in the material treated and in the method and details of procedure without departing from the scope of the invention as defined in the appended patent claims. I

Maleic anhydride is a well known organic chemical substance melting at about 52 C. and boiling at about 200 C. According to Beilstein, Vol. XVII (fourth edition) p. 433, it is converted exceedingly rapidly to maleic acid by water. Maleic acid on the other hand is converted to maleic anhydride and water upon heating at temperatures of about 130 C. or above.

Maleic anhydride is produced from organic compounds, in accordance with one known procedure, by passing the organic compound, for example, benzene, in the vapor phase mixed with oxygen, air, or other gas mixture containing free oxygen, through an enclosed space (such as a socalled converter tube) containing an oxidation catalyst, which is usually deposited upon a carrier. The resulting reaction mixture contains maleic anhydride. water vapor, nitrogen, C0, C02, and other products of combustion.

Heretofore, in the recovery of the maleic anhydride from such reaction mixtures, it has been the general practice to recover the maleic anhydride and the water vapor jointly in the form of maleic acid, for example, by absorbing the products of the reaction in water or by cooling the reaction'mixtures to condense the products including the water vapor; and, when maleic anhydride was desired, to then convert the recovered maleic acid to the anhydride by a separate dehydration treatment in the presence of dehydrating agents, such as phosphorus pentoxide.

I have found, accof' g to the present invention, when a gaseous nnxture containing maleic anhydride and water vapor, and particularly one resulting from the catalytic partial oxidation of an organic compound in the vapor phase, is contacted with a water-immiscible substance which is liquid under the conditions of treatment, that the maleic anhydride is absorbed and maybe directly recovered from the absorbent with high ill) eidciency. The water vapor is not absorbed to any substantial extent and passes ofl with the eilluent gases. As compared with the prior processes according to which the maleic'acid is recovered in the form of the acid by condensation of the re-= action products or absorption in water, and then converted to the anhydride by a dehydration process, the process of the invention effects a sawing in'the direct recovery of the maleic anhydride, and the elimination of a separate dehydration process.

I have furthermore found, in accordance with the present invention, when such gaseous reac= tion mixture resulting from the partial oxidation of an organic compound is treated in this manner, that the maleic anhydride is absorbed in preference to many of the impurities and consequently that the maleic anhydride may be sepa= rated from the absorbent in a purified condl= tion. T, have also foundas an additional Ieature oi the invention, that a purification of the malelc anhydride can be effected by dissolving it at a temperature above its melting point in a. solvent, permitting the solution to cool to effect crystallization, and separating the crystals.

in the practice of the present invention in accordance with a preferred method of procedure, a gaseous mixture containing maleic anhydride, such as the reaction mixture resulting from the catalytic oxidation of benzene or other suitable organic substance in the vapor phase, is passed through, or otherwise in contact with, a substan= tially inert liquid which is immiscible with water, preferably maintained at a temperature of an proximately 45 to 70 C. l'he maleic anhydride is absorbed and removed from the mixture and the unabsorbed portion of the mixture is then passed to a water scrubber where the residual products are condensed and removed. The maleic anhydride may be separated from the absorbent in any suitable mann r, as for example by cooling the solution to as low a temperature as practicable, permitting it to crystallize, and separating the crystcls'of maleic anhydride by filtration, centrifuging, or the like,

The possibility oi recovering maleic anhydride such by this procedure, notwithstanding the fact that the process is advantageously carried out at temperatures at which maleic anhydride and water ordinarily combine to form maleic acid (temperatures below 130 C), result from the combination oi several factors involved in the process; notably, the maintenance of the water in the vapor phase, and the removal of the maleic anhydricle from the reactive influence of the water. The invention in one of its broader aspects accordingly comprises condensingmaleic anhydride from a mixture of maleic anhydride and water vapors at a temperature at which maleic anhydride ordinarily reacts with water to form maleic acid, while retaining the water in the vapor form, and removing the maleic anhydride condensate from the reactive influence of the water vapor.

The employment of gaseous mixtures contain, in addition to the maleic anhydricle and water vapors, gases which do not react chemi sally with maleic anhydride under the conditions of the absorption (so-called inert gases) consti tutes an advantageous method of keeping the water in the vapor phase since the presence of the inert gases reduces the water vapor pressure of the mixture to a value below the vapor pressure of liquid water at the temperature to w co the vapor mixture is cooled, thereby permitting the co: tures at eol s me rly in connect out are employed.

The absorbent immiscible liquid. compound which is to the reaction mist o M. ,i to be recovered. is so is preierahly an organic rest with respect oh the product i and eirlciency lilowing ably more oi usher "chores, and. it

should also erties: it shou ,solve co the partial oral .oii aredo" tures than it does lower should it should be liquid ily posse.u

e liquid may sure e.- tively low vapor pressure, and liquids which boll 55,5 at a relatively low termlerature, 'Zor example,

' as well as those which boil at higher 2: moreover, that 9 r be secured with absorb= greater or less satisfactory l. ents which some but not a When a 2 also t is ill the ii-qt snip d as absoroe perature at which absorption is Cfill'lli with solubility of the product the absorb-- Bill. For economical commercial operation, em absorbent liquid ose cost is s1ib-= stantiai, a pressure may be em oyecl which confines the loss oi. absorbent in the exit gases to about 5 per cent, preferably about 2 per cent. l /"hen a liquid is employed; as absorbent which has a relatively high boillhg point (for example, above 256 C.) and/or Whiijil has a relatively low vapor pressure (for example a vapor pressure exceeding 25 mm. of ll'if"'ill"y at the tempera ture employed), or when the loss of solvent in the exit gases is not material, the absorption may carried out under atmospheric pressure conditions.

Among the absorbents which are in process, there may be mentioned, by way of ample: liquid hydrocarbons of the aliphatic aromatic series, others, are iatic KlilJTCO CO pounds, haloge" hyrlrocarlisohs of phatic and or 0 series, halogenated h serrated aromatic hydrocarbons, etc, tor-es thereof; as for example benzol, toluo vent naphtha, petroleum ether, gasoline, oil, heavy solvent, diphenyl, nitrobenzcl, llliZIO- toluene, mixtures of itrotoluenes, dl hyl ether,

the

.diphenvl oidiie, chloroform, carbon tetrachloride,

ehlorobenaenes, chlcronaphthalehes, etc.

The gaseous mixture containing the products to be recovered may be produced in any suitable manner, and if not at a suitable pressure may he 1. being into contact e absorbent. Hurthezlore, although varitures maleic aohvdricle vagoors containing watervanor may be treated in with the process of the present invention, t is preferable to employ a gas mixture an inert gas (such as air, G62, fi l). iitrogen, etc.) in such an amount that the gas ture will not be saturated with water vapor, and preferably not more than 50 per cent saturated with water vapor, under the temperature and pressure conditions prevailing during the absorption. Ordinarily, the gaseous reaction mixtures resulting from the catalytic partial oxidation contain a sufficient amount of inert gas for this purpose. If the gas mixture to be treated does not contain inert gas in sufficient amount, additional inert gas may be added as a diluent prior to or during the absorption of the maleic anhydride.

The temperature at which the absorption is carried out will vary with the liquid employed absorbent, with the pressure under which the ,Tcsorption is carried out, with the solubility in he lio'oicl employed of the product to be recov re and with the dew-point of the product in he gas mixture treated. A temperature is gena y employed which is below the dew-point or" he product to be recovered in the gas mixtore-,z-so that, in eiiect, a condensation of the product will take place in the solvent. Preferably temperature is employed which is conra ly, for example to 50 (1, below the point; which does not result in an unecoi solvent in the gas stream pass and at which the solo.

illustrative "'llililfli'lllhl'lts of manner in oh. the invention may be oracticed, the lol sales are presented. The parts are Example ifl Beneene (lienzol) is vaporized, the vapors are mixed with about times their Weight of air, and the mixture is passed through a converter tube containing a contact mass, comprising a mixture of vanadium and molybdenum oxides on a catalyst carrier, maintained at a temperature of about 5E0 to about 520 6., the

time of contact between the vapor mixture and catalyst being about 0.2 second. The gas cperature of about 116 to 150 C, and cooled through an absorption vessel which 5 baffles, bubbler plates, or similar do" curing an intimate contact between liquid, and which is partly filled maphthalene maintained by suit ling means at a temperature of about alpha-chlornaphthalene becomes maleic anhydride, it is withdrawn rber cooled, preferably to A mass of small, fine, .rnaleic anhvclride set i cir stals are separated from ..ple, by filtration or nfy the znaleic anhycl with about a plat... heated to reupon the '0 into solu not solution is c acid or other solvent to remove insoluble impurities, ii necessary, and then slow-- l cooled to about room temperature to eiieot crystals of maieic h a small amount ci' ed. iaaIeic arihjyd is oi? sanity oi from about i oer c t.

The alpha=chlornaphthalene mother-honor resulting cry tallization stepcan be returned to the absorber for reutilization in the process. The alphaclilorhaphthalene mother liquor can be repeatedly used in the absorption process, the amount lost being replaced by addition of alpha-chlornaphtha1ene, since the surprising observation has been made that there is a limited accumulation of impurities in the absorption liquid above which there is no substantial increased accumulation. The solvent naphtha mother-liquor remaining from the purification step can be reused in carrying out further purifications until the accumulation of alphachlornaphthalene therein becomes excessive, ai-ter which it can be subjected to distillation to separate and recover the solvent naphtha from the residual aioha-chiornaphthalene containing maleic anhydride and impurities. The recovered solvent naphtha can be reused in carrying out subsequent purifications, and the residual alphachlornaphthalene can be added to the absorbing liquor and reused. It will thus be evident that substantially all of the maleic anhydride produced is recovered, with the exception of that passing out of the absorber in the gas stream. The gas stream leaving the absorber and containing unabsorbed rnaleic anhydride, maleic acid, and other products of combustion, is passed through absorber or absorbers containing Water from which absorbed material can he re covered as an *npure solution of maleic acid. Example 2. .li gaseous reaction mixture containing maleic anhydride and resulting from the vapor phase catalytic oxidation of benzene or other suitable organic substance, such as the cooled reaction mixture treated in Example 1, is bubbled through an absorption vessel which is partly filled with solvent naphtha held at a temperature of to C., while maintaining a superatmospheric pressure of i5 to 20 lbs. per square inch. The gases leaving the absorber are passed into a second absorber partly filled with solvent naphtha maintained at a temperature of 23 to i5 i3. and at a superatmospheric pressure of 15 to 20 lbs. per square inch. The absorption vessels are equipped with suitable cooling means, with names, bubbler plates or similar devices for assuring an intimate contact between the gas and liquid, and with suitable means for maintaining the superatrnospheric When the solvent naphtha in the first absorber absorbed from about one hali to about six times its own wei ht of maleic anhydride it is Withdrawn from the absorber. The rnaleic am hydride is separated from the solvent naphtha cooling the mixture, preferably to about 3 to produce crystallization of the maleic an hvdride, and the crystals are separated from the suitable manner.

he resulting crystals may be glorified ii ricrecrystal ization solvent naphtha,

as disclosed in Example 1.

pressure.

liquor by filtration, centrifuging, of. in l A the course of the operation. solvent taining some maleic anhydride, maleic acid, other products of oxidation and solvent naphtha, are passed through an absorber, or absorbers, containing water maintained by suitable cooling means at a temperature of about 10 C., whereby residual products of oxidation and considerable quantities of solvent naphtha carried by the gases are absorbed and condensed. The solvent naphtha, after separation from the aqueone solution and drying in any suitable manner, for example by distillation, may be reused in the process. The absorbed residual oxidation products may be recovered as an impure solution of maleic acid. Any solvent naphtha remaining in the gases leaving the water absorbers may be recovered by absorption in a suitable relatively non-volatile liquid absorbing agent for the solvent naphtha, as for example straw oil; for example by passing the gases through a tower or towers filled with packing rings or other inert surface-giving packing material and through which the liquid absorbing agent is circulated. The solvent naphtha may be recovered from the absorbing agent by distillation, and reused in the process. The solvent naphtha may also be recovered from the effluent gases by treatment with solid absorbing agent, such as silica gel, activated carbon, etc.; for example by passing the gases through a tower or towers packed with a-suitable granular absorbing agent from which the solvent naphtha may be recovered for reuse by heating.

Instead of separating the maleic .anhydride from the absorbing liquid by cooling the solution thereof in the absorbing liquid, the solution may be subjected to fractional distillation, at either atmospheric or sub-atmospheric pressure, and the maleic anhydride recovered separately from the absorbing liquid, which may be reused in the process.

his to be understood that the invention is not limited to the above examples, and that the various proportions and conditions cited are all capable of wide variation.

The ratio of benzene to air employed in the oxidation process may be varied within wide limits; for example, ratios of about 4 to 50 parts by weight of air to one part by weight of benzol maybe employed; and instead of air, oxygen, ozone, or other oxygen-containing gas mixtures may be used. The reaction temperature also may vary from about 375 to 550 C., and the time of contact may be irom about 0.10 to 2.9 seconds. Furthermore, pressures at, above, and below atmospheric may be used in carrying out the oxidation.

In recovering the maleic anhydride from the reaction mixture, cooling of the gas stream leaving the converter is not essential, but it is pre fer-red" since it enables operation of the absorber under more economical conditions. The absorber may .be constructed in any suitable 'manner known to the art, and may consist of a single unit (such as, a tower filled with packing rings or other inert surface-giving packing material, or a vessel containing bubbler-plates, bailies', or the like), or it may be composed of a number of such units in series. The absorbing liquid may be circulated through the absorber, countercurrent to the gas stream, or otherwise, so as to act as a washer or scrubber for said stream, or the gas may be bubbled through a stationary body or bodies of absorbing liquid. Furthermore, the absorption may be made continuous or semi-continuous in any well-known manner, for

example, properly adjusting the quantity and rate of flow of the circulating absorption liquid (if the circulating type of absorber is used) so that a saturated solution of maleic anhydride will be continuously formed and removed; or by removing the liquid when it becomes saturated,

stream and jointly condensing said vapors with the maleic anhydride while leaving the water vapor and other undesired constituents of the gaseous mixture uncondensed.

The temperature at which the absorption is caused to take place may vary within wide limits, although temperatures of 45 to 70 C. are preferred; since for any given vapor mixture the amount of maleic anhydride absorbed per unit weight of absorbing liquid decreases with increase in temperature. Thus, at temperatures of about 110 (3., the amount of maleic anhydride absorbed from the gaseous mixtures employed in the above excl pies is very small. Temperatures below 45 Q. also be employed, but it has been found that at such temperatures there results a decrease in the amount of maleic anhydride recovered from the gaseous mixtures employed in the above examples; for at temperatures of about 40 *C. or lower; water condenses to some extent converts a portion of the anhydrldc to maleic acid. It is noted that with a reaction mixture and operating conditions such as are above described, and with absorption temperatures 01' about to about 70 C., the ab sorption takes place satisfactorily, and more over the water-vapor present in the gas stream has no substantial hydrating effect upon the maleic anhydride.

Instead of separating the maleic anhydride from its solution in the absorbing liquid by cooling the solution, the absorbing liquid may be removed from the maleic anhydride by fractional distillation and recovered separately, as described above in connection with Example 2. The recovered absorbing liquid may be reused in the process.

While in the preferred practice of the invention the maleic anhydride is absorbed in an inert water-immiscible liquid to remove it from the reactive influence of the water vapor present in the vapor mixture subjected to treatment, the process also can be carried out without the use of such absorbent. Thus, a vapor mixture containing maleic anhydride and water vapors can be cooled below the dew-point of the mixture with respect to maleic anhydride, while retain ing the water in the vapor form, the resul maleic anhydride condensate and the rein. ing uneondensed vapors can be separately with drawn, and the maleic anhydride condensate collected in a receiver adapted to hold it out of contact with the remaining vapors. For exam 'ple, the gas mixture of the above examples can be cooled to about C. by contact with a cooling surface and the resulting liquid condensate run off, as formed, into a receiver while separately drawing off the remaining uncondensed vapors.

. Furthermore, in the above purification process, 7

. essence crude crystals of maleit anhydride may be portions, but may be varied within considerable limits, since molten maleic anhydride is substantially miscible in all proportions with some solvents, as for example, solvent naphtha. The proportions of solvent above given are preferred, however, for economical operation and ease of handling, as they provide a fluid slurry of crystals and mother-liquor upon crystallization. The temperature at which the solution is eflected preferably should m above the melting-point of malelc anhydride, but it preferably should not exceed about 70 6.; inasmuch. as it has been found that at higher temperatures maleic acid, which sometimes may be present in small amounts, tends to go into solution in the molten maleic su'il-lyclride. 28y efiecting the solution with molten rnaleic anhydride, the amount of solvent required, and consequently the amount of maleic anhydride remaining in. the mother-liquor, is greatly decreased.

While the process of the invention has been described with particular reference to the recovery or maleic anhydride from gaseous mixtures, and particularly those containing it'together with water vapor, the process is not limited thereto but may be employed for the recovery of other organic compounds which react with water. For example, the process is applicable to the recovery of anhydrides of other organic polycarbcxylic acids from gaseous mixtures contaim ing the same, particularly gaseous mixtures which also contain water vapor, and especially gaseous reaction mixtures resulting from the vaponphsse catalytic oxidation of organic compounds; as for example, phthallc anhydride, naphthalic anhydride, etc. The process also may be employed for the separation and recovery of a plurality of partial oxidation products from a gaseous mixture containing them. Thus, for ex ample, phthalic anhydride and maleic anhydride may each beseparately recovered from a gaseous reaction mixture resulting from the vapor-phase catalytic partial oxidation of naphthalene or other organic material by passing the gaseous reaction mixture in contact with a suitable absorbent maintained under temperature and pres sure conditions suitable for the absorption of phthalic anhydride and unsuitable for the ab-' sorption of the other products contained in the gaseous mixture (for example, diphenyl oxide maintained at a temperature oi about 85 to about 105 C.) then passing the residual gaseous U mixture in contact with a suitable absorbent molnteined under temperature and pressure conditions suitable for the absorption of rnaleic anhydride but not other constituents of the gaseous rrdziture, such as water and carbon dioxide (as for example, alphawhloronaphthalene maintained at a temperature of about 45 to about dii and recoveri th phthalic anhydride and rseleic anhyciri their respective solutions. 7 The inventio. s furthermore not'liinited to or" the substances are specifically disc bu includes equivr rill dissolve the organic mutation products und r the indicated temperat me pressure ions. For example, submay be employed absorbents which are normally solids but which are liquids under the absorption conditions, as for example, naph thalene, particularly crude naphthalenes; and is intended to include such substances within the scope of the expression a liquid in this description and the claims. Furthermore, other normally liquid absorbcnts may be used; such as esters of monoor poly-carboxylic acids (for can ample diethylor dibutyl-phthalate or maleate. etc), particularly under absorption conditions which preclude the condensation of substantial amounts of water.

It will be understood that the term catalytic maleic anhydrid'e, as used in the claims, refers to and is limited to a maleic anhydride which has been prepared by the partial or selective oxidation of an organic body, as for example benzene, in the vapor phase by means of a gas containing free oxygen in contact with a suitable catalyst.

Since, in the practice of the invention, changes may be made in the details of the above disclo' sure and in the process above described without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrating, and not limiting, the invention, except as limited by the claims.

This application is a continuation-in-part of my application Serial No. 595,431, filed February 26, 1932, which is in part a continuation of my application Serial No. 297,091, filed August 2, 1922 ,for Production of maleic anhydride, issued as United States Patent 1,395,522 on January 31, 1933..

I claim:

l. The method of recovering maleic anhydride from a mixture containing maleic anhydride and water vapors, which comprises condensing maleic anhydrlde from said vapor mixture at a term perature at which maleic anhydride reacts with water to form maleic acid, while retaining in the vapor form the water in the vapor mixture by maintaining the water vapor pressure of said vapor mixture below the vapor pressure of liquid while maintaining the water vapor pressure of said vapor mixture below the vapor pressure of liquid water at said temperature, whereby maleic anhydride is separated from said vapor mixture in a, non-vapor form while the water is retained.

in the vapor form, and removing the resulti maleic anhydride from the reactive influence or the water in the vapor form.

3. The method of recovering maleic anhydride from a mixture containing maleic anhydride and water vapors which comprises condensing malelc anhydride from said vapor mixture at a tern perature at which maleic anhydride reacts with water to form maleic acid, in the presence of suincient inert gas to retain the water in the vapor form, and removing the maleic anhydride condensate from the reactive influence of the water vapor.

i. The method oi recovering maleic anhydride from a gaseous mixture which is at an elevated temperature and which contains maleic onhyciride and water vapors and an inert gas, which comprises cooling the gaseous mixture to a temperature below the dew-point of the mixture with respect to maleic anhydride, to condense maleic anhydride, while having present an amount of inert gas not substantially less than that required to retain the water in the vapor form, said temperature also being sufliciently low to condense the water vapor in the absence of the inert gas under the existing pressure conditions, and removing the maleic anhydride condensate from the reactive influence of the water vapor.

5. The method of producing maleic anhydride which comprises forming a mixture of air and vapors of an organic substance which upon oxidation produces maleic anhydride, passing the mixture in contact with an oxidation catalyst under conditions adapted to produce maleic anhydride, absorbing maleic anhydride from the resulting gas mixture in an inert water-immiscible organic liquid which is a solvent for maleic anhydride maintained at a temperature of about to about 0., and recovering maleic anhydride from the resulting solution.

6. The method of producing maleic anhydride which comprises forming a mixture of air and vapors of an organic substance which upon oxidation produces maleic anhydride, passing the mixture in contact with an oxidation catalyst under conditions adapted to produce maleic anhydride, and absorbing maleic anhydride from the resulting gaseous reaction mixture in an inert waterimmiscible organic liquid which is a solvent for maleic-anhydride maintained at a temperature of about 45 to about 70 C., the amount of air employed. in the oxidation being such that the dew-point of the gaseous reaction mixture with respect to water is below said temperature.

7. The method of producing maleic anhydride which comprises forming a mixture containing a to 20 parts by weight of air per part by weight of benzene, passing the mixture in contact with an oxidation catalyst under conditions adapted to produce maleic anhydride, absorbing maleic anhydride from the resulting gas mixture at a tem perature of about 45 to about 70 C. in an inert water-immiscible organic liquid which is a sol vent for maleic anhydride, and recovering maleic anhydride from the resulting'solution.

B. The method of recovering maleic anhydride from a gaseous reaction mixture produced by forming a mixture of air and vapors of benzene and passing the mixture in contact with an oxidation catalyst under conditions adapted to pro duce maleic anhydride, which comprises absorbing maleic anhydride from the gaseous reaction mixture inalpha-chlornaphthalene, crystallizing maleic anhydride from the resulting solution, separating the crystals from the mother liquor and recrystallizing the crystals from a volatile solvent. I

9. The method of recovering maleic anhydride from a gaseous reaction mixture produced by forming a mixture containing about 25 parts by weight of air per part by weight of benzene, and passing the mixture in contact with an oxidation catalyst under conditions adapted to produce maleic anhydride, which comprises absorbing maleic'anhydride from the gaseous reaction mixture in alpha-chlor-naphthalene, crystallizingv maleic anhydride from the resulting solution, dissolving the resulting maleic anhydride in molten condition in a solvent therefor, and. crystallizing maleic anhydride from the resulting so lution.

iii. if

adapted. to a s c prises absorbing maleic a hydilde cm the gaseous reaction mixture olvent naphtha main tained at a temperature of 70 0., and recovering rnaleie resulting solution.

11. In the production i enliyt rltile the step which comprises di solving molten maleic anhydride in a solvent 12. In the production or purified condition, the ste solving molten catalytic to temperature not above "10 o solvent therefor, and separati from the resulting solution.

13. The method of recover is the anhydricle oi an organic polycarhoxylie from a mixture containing vapor the anhydride and water vapor, which comprises condensing the auhytlride in the presence of an inert water-=immlscible sol vent therefor while retaining the water in the va=- rich comprises dic cic anhydride at a in an inert volatile nialelc anlwc'iride per form.

14.. The me of recovering maleic anhyrlride from a mix -itaining maleic anhydririe anti water vapor uch comprises condensing maleic .sence oi an inert water irn while retaining thewaanhyclrirle i r miscible solvent oi'iEi eilor ter in the vane;

15. The mother from a dride and water and an i. comprises condensing ti presence oz"; can wate' therefor while the water gas mixture.

16. the method oi. reeoverirfrom a mixture containing maleic anhydritle anti water vapors, which comprises condensing maleic anhydride in the presence of inert water ira n ciole solvent therefor at a temperature below ti coiling point oi water under the existing pre conditions While retaining the water in vapor form.

1?. The method of recovering maleic esihytlfltle from a gaseous mixture containing malelc s by" driole and water vapor nd an inert gas, which comprises condensing maleic anhydride in the presence of an inert water-in1rniscihle solvent therefor at a temperature below the point oi water under the existing pressure conditions while retaining the water vapor in the gas mire ture by means or said inert gas.

18. The method of recovering rnaleic ering maieic from a gaseous mixture which is at an elevated eic anhydride in a u immiscible organic liquid which is a solvent for maleic anhydride.

19. The method of recovering rnaleic amiydridc l to at an elevated .iture and which c itains maleic anhy crater vapors and inert gas, which rises contacting the gaseous mixture with an t organic liquid which is a solvent for maleic anht ride and which is at a temperature below the etc-point of the mixture with respect to maieic anhyclride but above the dew-point of the mixture with respect to water, said temperature also being sufficiently low to condense the water vapor in the absence of the inert gas under the existing pressure conditions, whereby a solution of inaleic anhydride in the solvent is produceci.

20. The method of recovering the anhydride of an organic polycarboxylic acid from a gaseous mixture containing vapor of the anhydride, water vapor, and an inert gas, which comprises condensing the anhydricle in the presence of an inert svater in'nniscible solvent therefor while retaining the water vapor in the gas mixture.

The method of recovering the anhydride of an organic polycarboxylic acid from a gaseous mixture containing vapor of the anhydricle, water vapor and an inert gas, which comprises absorbthe anhydride in an inert water-immiscible solvent therefor at a temperature below the dewpoint of the gaseous mixture with respect to the anhydricie in the presence of sufficient inert gas to retain the water vapor in the gas mixture.

22. The method of recovering the anhydride of organic dicarboxylic acid which is a solid under normal conditions of temperature and pres sure from a gaseous mixture containing vapor or" 1 the anhydricle, water apor and an inert gas,

which comprises absorbing the anhvdride in an inert water-immiscible liquid which is a solvent for said anhydride maintained at a temperature below the dew-point oi" the gaseous mimure with respect to the anhydride and above the dew-point oi the gaseous mixture with respect to water.

The method of recovering maleic anhydride from a mixture containing maleic anhydride and water vapors, which comprises condensing the I maleic anhydricle in the presence of an inert water-immiscible liquid which is a solvent for maleic anhydricle while having present an amount of inert gas not substantially less than that re quired to retain the water in the vapor iorm.

The method of recovering rnaleic anhydrlde a gaseous mixture containing maleic anand water vapors and an inert gas, which it arises condensing the maleic anhydriole'in the presence of an inert water-immiscible liquid which is a solvent for inaleic anhvdride, at a temperature below the boiling point of water i the existing pressure conditions and below the dew-point oi the gaseous mixture with respect to malcic anhydricle while substantially retaining the water vapor in the gas mixture.

25. The method of recovering maleic anhydricle from a gaseous mixture containing inaleic anhv and water vanors, which comprises abscrh said maleic anhydricle in an inert wateri "cihie organic liquid which is a solvent 1 r c anhvdride maintained at a tempera" e noini; of water under the fess than that required to retain the wat r the gas mixture.

The or recovering maleic anhyriride, which comprises passing a gaseous mixture containing maleic anhydride and water vapors and an inert gas into contact with an inert water= immiscible organic liquid which is a solvent for maleic anhydride maintained at a temperature below the boiling point of water under the existing pressure conditions and below the dew-point of the gaseous mixture with respect to maieic anhydride and above the dew-point of the gaseous mixture with respect to water.

27. The method of recovering maleic anhydride from a gaseous mixture containing catalytic maleic anhydride and water vapors and an inert gas, which comprises aborbing said maleic sinhydride in an inert water-immiscible l quid which is a solvent for maleic anhydricle maintained at a temperature below the boiling point of water under the existing pressure conditions and below the clevwpoint of the gaseous mixture with respect to maleic anhvtiride and above the dewpoint oi the gaseous mixture with respect to water, whereby maleic anhydride is condensed while water vapor is retained. in the gas mixture.

28. The method of recovering maleic anhydride, which comprises passing a gaseous mixture containing maleic anhydrlde and water vapors and aninert gas into contact with alpha-chlornaphthalene maintained at a temperature oi about to about 60 C. in the presence of an amount of inert gas not substantially less than that required to retain the water vapor in the gas mixture, and recovering maleic anhydride from the resulting solution.

$9." The method of recovering maleic anhydride train a gaseous reaction mixture produced by forming a mixture of air and vapors or" an organic substance which upon oxidation produces maleic anhydride, and passing the mixture in contact ,with an oxidation catalyst under conditions adapted to produce maleic anhydride, which "comprises condensing maleic anhydride from the gaseous reaction mixture in the presence of an inert water-immiscible organic liquid while substantially retaining the water vapor in the gas mixture.

30. The method of recovering maleic anhydride from a gaseous reaction mixture produced by forming a mixture containing i to parts by weight of air per part by weight of an organic substance which upon oxidation produces inaleic anhydride, and passing the mixture in contact with an oxidation catalyst iuider conditions adapted to produce maleic anhydririe, which com prises absorbing maleic anhvdride from the gase ous reaction mixture in an inert water-immiscible organic liquid which is a solvent for maleic anh vdride and which is maintained at a temperature below the boiling point of water under the existing pressure conditions and below the dewooint or" the gasecus reaction mixture with reect to inaieic anhydride and above the dewil lt oi the gaseous reaction mixture re act to water, whereby maleic anhvdride is condanced while water vapor is retained in the gas 

